Power supply in three phases is the standard for a modern country house, garage with professional equipment or workshop. Tension. 380 volts. It allows you to connect powerful consumers, such as welding machines, three-phase electric motors or heating systems, without critical network overloads. However, the ability to connect more power requires a more responsible approach to protection.
The main task of the circuit breaker is to protect not the equipment, but the equipment itself. cable. The wiring warms up when the current passes, and if the current exceeds the permissible norm, the insulation will begin to melt, which will inevitably lead to a short circuit or fire. That is why the choice of the nominal value of the machine is always tied to the cross section of the vein, and not to the power of the connected device.
Errors in calculations in three-phase network are more expensive than in single-phase, as the currents are higher, and the consequences of accidents are larger. In this article we will analyze how the cross section of copper and aluminum veins relate to the nominal current of protection, and why you can not just take a machine with a margin.
โ ๏ธ Attention: If your project involves connecting an electric boiler or charging station for an electric car, be sure to specify the current requirements of local power supervision. The rules of load distribution and power limits may vary depending on the technical conditions of a particular settlement.
Principles of operation of the automatic switch
An automatic switch, or simply an โautomaticโ circuit breaker, is a complex electromechanical device. It responds to two types of threats: short circuit and overload. For the first is the electromagnetic sever, which works instantly with a sharp jump in current. For the second - a thermal disengagement, which is heated proportionally to the current of the load and breaks the chain with prolonged excess of the norm.
The key parameter that we are interested in when choosing cable protection is the following: nominal (In). This is a value that the machine can pass through itself indefinitely without turning off. However, there is also the concept of โfailure currentโ and โoperation currentโ. According to the standards, the machine can not be turned off at currents up to 1.13 from the face value for an hour, but must work at 1.45 from the face value.
It is this feature that dictates the main rule: the nominal current of the machine must be less than or equal to the maximum permissible current for a given cable cross section. If you put the machine on 32A wire, which holds only 25A, then at 30A current the wire will already warm and degrade, and the machine will not even think to turn off.
- ๐ Heat disengagement protects against overload and works on the principle of a bimetallic plate, which bends when heated.
- โก Electromagnetic detachment protects against short circuit and works in fractions of a second with multiple excess current.
- ๐ก๏ธ Temperature correction: in hot shields or when installing several machines in a row, their nominal load may decrease.
The dependence of current on the cross section and material lived
The throughput of the cable directly depends on the cross-sectional area of its veins and the material from which they are made. Copper conducts current better than aluminum, so copper wire of the same cross section withstands a greater load. This is especially important for a 380V three-phase network, as even a small drop in the cross section can lead to significant heating.
The calculation must take into account the method of laying. A cable lying in the ground or monolithic in a wall is cooled worse than a wire hanging in a tray or open space. Therefore, for hidden wiring, often choose a section with a margin or reduce the nominal value of the machine. Also plays a role the number of lives in the beam: the tighter the wires are stacked, the worse the heat sink.
There are special tables of PUE (Rules of the Device of Electrical Installations), regulating the permissible current loads. For household and semi-professional needs at 380V, copper cables with a cross section from 4 to 16 mm2 are most often used. Aluminum is now used less often, mainly for large-section introductory lines, where weight and cost savings are important.
Why is aluminum worse than copper?
Aluminum has a higher resistivity, which requires an increase in the cross section of the vein to pass the same current. In addition, it is fluid (the contacts are weakened) and oxidized in air, forming a dielectric film, so it requires special terminals or lubricants.
โ ๏ธ Attention: Never connect copper and aluminum wires with a twist! At the point of contact, a galvanic pair will appear, oxidation, heating and eventual fire will begin. Use only special transition terminals or washers.
Table of correspondence of cable cross-section and nominal of the machine
For quick selection of protection, it is most convenient to use a summary table. It is made taking into account standard operating conditions and popular materials. The data are relevant for copper cables with PVC insulation (e.g., VWGng-LS) and aluminium analogs when air-laying or boxes.
| Section of the vein (mm2) | Materials | Permissible current (A) | Recommended automatic (A) | Max. power (380V) |
|---|---|---|---|---|
| 2.5 | Copper. | 25-27 | 16 | ~10 kW |
| 4.0 | Copper. | 35-38 | 25 | ~15 kW |
| 6.0 | Copper. | 45-50 | 32 | ~20 kW |
| 10.0 | Copper. | 60-65 | 40-50 | ~30 kW |
| 16.0 | aluminum | 60-65 | 40-50 | ~30 kW |
Please note that the table shows the denomination of the machine, which less maximum current cable. This is done to create a buffer zone. For example, a 4 mm2 cable holds a 36A, but the machine is placed on a 25A. This ensures that during prolonged operation at the limit (for example, 30A), the cable will not overheat, although the machine may not knock out immediately.
When choosing a cable for a three-phase engine, consider the initiation currents. They can be 5-7 times higher than the nominal, so the characteristic of the machine should be โDโ, not the standard โCโ.
Power calculation for three-phase network
Often users know the power of their equipment (such as an electric boiler or machine), but do not know what current will be consumed from the network. For a single-phase network, itโs simple: divide Watts by 220. But for 380 volts, the formula is more complicated, as current flows in three phases.
The basic formula looks like this: I = P / (โ3 ร U ร cos ฯ). Here. P - Wattage power, U voltage (380V), and cos ฯ - power factor. For active load (heaters, lamps), cosine phi is 1. For motors and pumps, it is usually 0.7โ0.8, which increases the current consumed.
To simplify calculations, electricians often use the rule of thumb: 1 kW of power in a 380V three-phase network consumes about 2 Amps of current. This is a rough approximation, but it allows you to quickly estimate the order of the numbers. Accurate calculation is necessary for the selection of the cable cross-section to avoid a voltage drop in long sections.
โ๏ธ Pre-installing check
Characteristics of detachment: B, C and D
The choice of the machine is not only the selection of the number of ampere, but also the understanding of its temporal characteristics. On the body of the device, in addition to the nominal value, there is a Latin letter: B, C or D. It shows how many times the current must grow to work instant protection against short circuit.
For the household network and most cables on 380V (outlets, lighting, boilers) used characteristic C. It works at a short circuit current exceeding the nominal 5-10 times. This allows you to survive short-term current surges when turning on devices without false shutdowns.
Characteristics B It is more sensitive (3-5 denominations) and is used in older networks or to protect lines with very long cables, where the current of the CC may be small. Characteristics D (10-20 denominations) are needed exclusively for electric motors with heavy start, so that the machine does not knock out at the time of start.
For standard wiring around a 380V home or garage, always choose machines with a C characteristic unless you have specific industrial equipment with large starting currents.
Common mistakes in choosing protection
The most common mistake is to choose a machine โunder loadโ, ignoring the cable cross section. People buy a powerful machine, put a 50A machine, and the liner is made with a 25A wire. The result: the wire melts inside the wall, and the machine is silent, because 30A for it is the working mode.
The second mistake is the use of an aluminum cable with an under-section paired with a copper machine. Aluminum has the property of โswimmingโ under the pressure of contacts. If you do not pull the connections regularly or do not use spring terminals, the contact will weaken, begin to warm and melt the body of the machine.
The third mistake is the installation of automatic machines with impaired calibration. Cheap Chinese devices may not hold the claimed current. At the professional level, it is recommended to use products of well-known brands, such as: ABB, Legrand or IEK The pros in the series where the claimed 16A really means 16A.
โ ๏ธ Attention: If you smell plastic or hear a buzzing sound when working the machine, turn off the load immediately. This is a sign of overheating of contacts or internal malfunction of the device, which can lead to a fire.
FAQ: Frequently Asked Questions
Can I put a higher denomination machine if the cable is warm?
Absolutely not. If the cable is heated, then its cross section is not enough for the current load. An increase in the nominal value of the machine will cause the protection to fail, and the insulation of the cable will melt, causing a short circuit. We need to change the cable to a thicker one.
Which machine to choose for a three-phase boiler 12 kW?
The current consumption will be about 18-20A. Taking into account the reserve and initiation currents of the pump, the 25A automatic machine with the characteristic โCโ is optimally suitable. Cable is better to use a copper section of 4 mm2.
What is the difference between a 380V single-pole and three-pole machine?
For a three-phase 380V network, three-pole (3P) or four-pole (4P) automatic machines are necessarily used. They break all phases at the same time. The use of three separate single-pole machines is prohibited, since in case of an accident in one phase the other two will remain under voltage.
Do I need to put an RCD after the machine?
Yes, to protect people from electric shock, the installation of ultrasound (difrele) is mandatory, especially in wet rooms or when using a power tool. The machine protects the cable, and the RCD protects the person.